1. Technical Field
The invention relates to the field of electronic audio signal amplifier and loudspeaker system design and, more particularly, to an electronic audio signal amplifier and loudspeaker system capable of omni-directional sound enjoyment and low intermodulation distortion.
2. Description of the Relevant Art
Electronic audio signal amplifier systems are typically designed for delivering maximum power to an electrical load, namely a loudspeaker. In the design of such a system, there is no particular attention paid to the delivery of acoustic energy to a human listener and, more particularly, to how that human listener perceives the delivered acoustic energy representative of an originally recorded audio sound.
To the contrary, it is typical to design an electronic audio signal amplifier system separately from the design of a loudspeaker system. Design is accomplished from a component approach such that one component is individually designed without particular regard to the specific design of another component which is coupled to it. Rather each component is designed separately to interface with a presumed component design. The result of this design approach may be, for example, the connection of a high power amplification system to a loudspeaker system which is not capable of handling the high power audio output signal levels. When the gain of the amplifier system is turned up, loudspeakers of the loudspeaker system may be irreparably damaged.
The perceptive capability of the human ear to receive sound energy has been scientifically analyzed by audiologists. It has been determined that the capabilities of men and women for perceiving sounds at different frequencies differ. Men, for example, are capable of listening to low frequency acoustic audio signals better than women, that is, in a frequency range beginning at approximately 20 Hz. On the other hand, women have an appreciably better ear for high frequency acoustic audio signals up to approximately 20 KHz. Furthermore, on the average, both men and women are able to hear and so appreciate low frequency acoustic energy better than high frequency acoustic energy. Also, the intelligible quality of acoustic energy which permits a listener to differentiate a violin, for example, from a cello relates to the low to mid range frequency response characteristics of the original sound which is that response region in which both men and women perceive sounds almost equally. Yet, the design of audio signal amplification systems including loudspeakers for acoustic energy transmission has for the most part ignored any system design perspective from audio electrical signal source to the human listener's ear.
Also prior art electronic audio signal amplifier systems, especially stereophonic systems, have evolved from the concept of intentionally building a soundstage from which direction acoustic energy emanates to a listener. Once a soundstage is built of speakers placed at particular locations or positions in a typical room, listeners also must carefully position themselves at a location to properly appreciate, for example, a left channel of directional stereophonic sound from a right channel of directional stereophonic sound.
Stereophonic loudspeaker assemblies are typically directional, i.e., they intentionally direct a sound toward an assumed listener position. Furthermore, the assemblies for stereophonic loudspeakers often comprise plural tweeters, woofers and subwoofers each of which may receive a full audio range of frequencies at its input such that there is a capability for intermodulation distortion from providing too great a frequency range and so an overlapping frequency range to each speaker type. To alleviate this problem, loudspeaker manufacturers or manufacturers of loudspeaker assemblies include passive crossover networks within the loudspeaker or assembly enclosure to separate frequency bands to a limited extent. Nevertheless, acoustic energy broadcast from one speaker type intermodulates with acoustic energy from another and creates intermodulation distortion because, even in systems including passive crossover networks, frequency bands overlap considerably. Furthermore, at least two such assemblies are provided for left and right channels of stereophonic sound and their respective acoustic energy outputs intermodulate and creat distortion.
As already briefly described, some loudspeaker drive assemblies, for example, coaxially mounted tweeter and woofer assemblies comprise first order passive filtering means for separating frequency bands for the tweeter while the woofer receives a full range of frequencies. The frequency bands still overlap considerably and create intermodulation distortion.
A speaker balance control is often provided to alleviate the requirement that a listener be precisely located at one particular position in a room. For example, if a listener is located close to a left channel loudspeaker than to a right channel loudspeaker, the listener may adjust a balance control to decrease the amount of amplification of the left channel and to increase the amount of amplification of the right channel so that the listener perceives the left and right channels equally. However, the listener who adjusts the left and right channel balance from a central balance switch position may not have any appreciation for the original balance of the audio signal recording or source. Consequently, the listener will probably adjust the balance inaccurately. Furthermore, once adjusted for the one listener location, another listener located elsewhere will perceive the sound differently and will also not be listening to the originally recorded signal or the signal originally generated at a source.
Also a treble/bass control or other complex equalization system is often provided to overcome the effects of unequal acoustic output over the audible frequency range from loudspeaker assemblies. Such equalization settings are controlled by a user who may not know the originally recorded audio sound and have little or no ability to correct the perceived frequency response.
Through the development of electronic audio signal amplifier and speaker systems for automobiles and other close environments, listeners have begun to appreciate the desirability of what may be considered a more omni-directional sound system in which the sound surrounds the listener. The more advanced of these systems may be quadrophonic and comprise speakers for four sound channels and an individually powered subwoofer speaker having its own output amplifier providing a fifth sound channel for bass frequencies. In such systems, a single crossover network may be applied for frequency division and limited audio channel delivery. The crossover network generally comprises a passive resistive and capacitive filter network for separating the subwoofer band of interest without particular regard to the sharpness of filter response. In fact, generally, the several speakers besides the subwoofer are intentionally vibrated over their entire frequency range and generate considerably intermodulation distortion via overlapping frequency bands of interest with other provided speakers.
Intermodulation distortion relates to an electrical phenomenon whereby electrical signals at one frequency interfere with and create undesirable modulation products with another desired and different frequency. The modulation products amount to noise and undesirably distort the original audio signal. Intermodulation distortion is a measurement in percent by which the high frequency energy at one frequency is modulated by the electrical signals at a second frequency. Intermodulation distortion is exacerbated in an electronic amplifier system through stages of amplification. For example, intermodulation distortion noise created in a first amplifier is amplified by a second stage of amplification and so creates additional undesirable modulation products with other desired frequencies at the second stage. A discussion of intermodulation distortion in amplifier systems is found, for example, in Transmission Systems for Communication, published by Bell Telephone Laboratories, 1970, at pages 320-331.
Another phenomenon uncovered through more recent inventigations at Bell Telephone Laboratories has been identified as transmodulation distortion. In theory, transmodulation distortion noise is created above intermodulation noise products and represents frequency modulation products of different phase and harmonics.
In a loudspeaker assembly, intermodulation distortion is further created, for example, in the translation of electrical energy to acoustic energy and may be experienced as an excessive, undesirable vibration of the loudspeaker assembly. For known loudspeaker assemblies and depending on the degree of utilization of the possible frequency pass characteristic for an included loudspeaker, intermodulation distortion may be in the range of ten to twenty percent. Despite the intermodulation distortion, such loudspeaker assemblies may still demonstrate a relatively flat frequency response over the band of interest. As indicated above, intermodulation distortion is related to undesirable mechanical vibration translated to a loudspeaker enclosed by the enclosure, to driving a particular loudspeaker type over too broad a frequency range as well as to other causes. A brief discussion of the theory of intermodulation distortion as applied to the design of loudspeaker assemblies is found in an appendix to U.S. application Ser. No. 294,364, concurrently filed herewith.
Consequently, despite the closed environment of an automobile, known automobile loudspeaker systems still tend to exhibit a considerable amount of both directionality and intermodulation distortion. When the driver adjusts volume and balance controls for front and rear speakers and for left and right channels so that the driver is please with the perceived sound, a front or rear seat passenger may not only be displease with the perceived acoustic energy but may not be positioned in a similar situation to the driver of the automobile for appreciating the originally recorded audio signals.
Consequently, there exists a requirement in the art for providing an electronic audio amplifier and speaker system which has similar effect to listeners, no matter where they may be located in a room or other closed compartment. Acoustic energy perceived by the listener across the entire audio frequency range must be equal no matter where the listener locates themselves in relation to the speakers. The listener should not necessarily be able to determine from which speaker a particular sound originates. From a different perspective, there remains a requirement in such a system to permit as many loudspeakers as possible to be located anywhere in a listening environment or at any height level without particular regard to a requirement for building a sound stage. Furthermore, there exists a requirement in the art for reducing the effects of intermodulation distortion and the creation of undesirable modulation products and so limit the potential for appreciating the originally recorded sound.